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Facebook users can fuel a social spam botnet by providing verified apps' access tokens in exchange for "likes" and comments.

Cyberattackers are using access tokens for legitimate Facebook apps as vehicles to spread spam on the apps' behalf. How do they do it? By tricking Facebook users into handing over their tokens in exchange for "likes," comments, friends, and followers.

Researchers at Proofpoint discovered an API access token for a verified Facebook app was being used to fuel comment spam on other Facebook pages. In this case, the victim page belonged to a major media outlet and Proofpoint customer.

Comment spam both interferes with social media interactions and exposes users to phishing and malware. A lot of comment spam found on social channels is generated by botnets, as was the case here.

In this case, spammers used the official HTC Sense app to trick users by leveraging an outdated version of the app and an earlier version of the Facebook API. How it worked:

Spam comments on the company's page referred to websites containing instructions for installing a particular version of the HTC Sense Facebook app. Users clicked the links and installed the app. They were shown how to get a developer-level API access token for the HTC Sense app.

From there, users were asked to copy the access token and paste it into a third-party website, run by bad actors, in exchange for likes, comments, etc. Once they passed along the token, the website was fully able to automate comments and other actions on behalf of the users.

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Ali Mesdaq, director of digital risk at Proofpoint, says this has been going on "at the very least for several months," though he cannot say with certainty when it started. Researchers came across the spam while working to protect a large news organization.

"When we dug deeper into why they had such a rise in spam comments in the last several months, we uncovered this campaign," he explains. "A number of the spam comments referenced how to install the app on individual Facebook accounts."

Since this issue was discovered, Facebook has made changes to prevent end-users from accessing developer-level API tokens, and provided best practices for building secure applications. HTC has removed the problematic versions of its HTC Sense app.

It's worth noting that several apps other than HTC Sense were targeted. This is one of many ways social media spam poses a risk to brands by interfering with user interaction and diluting corporate messaging on social channels.

"The implications are that we are starting to see mass broad sophisticated campaigns of spam on social media," Mesdaq explains. "Social media should be considered a hot target for attackers, and we expect the volume, diversity, and intensity of attacks to greatly increase."

The promise of more likes, comments, and friends may sound appealing, but this is a wake-up call to remind users that granting permissions to apps, and handing over access tokens, can lead to a spam overload, explains Proofpoint in a blog post. This can result in account suspension.